function cont(fname,nx,ny,lx,ly)
%function cont(fname,nx,ny) reads input file fname and makes contour plots
%of data of dimensions (nx,ny) on grid -lx/2 < x < lx/2 and -ly/2 < y <
%ly/2
 dx = lx/nx; dy = ly/ny;
 [X,Y] = meshgrid(-0.5*lx + (0.5:nx-0.5)*dx, -0.5*ly + (0.5:ny-0.5)*dy );
 A = load(fname);
 B = reshape(A(:,4),ny,nx);
 [C,h] = contour(X,Y,B,4);
 %axis([-0.6 0.6 -0.6 0.6])
 %legend('Streamlines center vortex',3)
 text_handle = clabel(C,h);
 set(text_handle,'BackgroundColor',[1 1 .6],...
    'Edgecolor',[.7 .7 .7])
 colormap cool
end